CENTRIPETAL FORCE

Students put rubber stoppers on the ends of fishing line.  The line runs through  a piece of glass tubing.  Students spin the stoppers around and try to keep them moving at a constant radius.

Before doing this lab, students are asked a question:  If you spin one stopper around with one hanging from the bottom, and then spin one stopper around with two hanging from the bottom, how will the period change?

Students always guess that the period will be cut in half.  However, their predictions are always a bit off.


F= m4p²r / T²

This equation gives centripetal force.  Since we are doubling the number of hanging stoppers, this is like changing "F" from one to two.
Since we are not changing the mass of the swinging stopper, we can just treat the "m" as the number 1.
Lastly, the 4p²r also remains unchanged, so it can also be treated as the number 1.

So the revised equation looks like this...

With one hanging stopper: 1=1/T²

With two hanging stoppers: 2=1/T²

If we solve these equations for "T", we get T=1 and T=.71 This relationship always shows up.  Students get ratios like 5:3.